The production and recovery of hydrogen by steam reforming of methane-rich gas mixtures, such as natural gas, naphtha, or other mixtures of low molecular weight hydrocarbons, are well known in the art. Processes are also known in which the effluent from hydrocarbon steam reforming is subjected to a secondary oxidative operation in the presence of added air. In instances where the produced hydrogen is intended for use in production of ammonia, the amount of air employed in the secondary reforming step serves, in certain of the known systems, to furnish all or part of the nitrogen required for reaction with the product hydrogen for ultimate synthesis of NH.sub.3. Since oxides of carbon (CO and CO.sub.2) are present in the secondary reformate as undesired contaminants, the secondary reformate is subjected to water gas shift reactions to convert CO to CO.sub.2 and then to procedures for removal of contained CO.sub.2 and for conversion of any residual carbon oxides to methane (methanation). This conventional process for production of ammonia synthesis gas is described, for example, in U.S. Pat. No. 4,479,925 and in Kirk-Othmer: Encyclopedia of Chemical Technology, third edition (1978), volume 2 pages 470-516; a typical process flow diagram appearing at FIG. 13 on page 488.
The gas composition obtained by the primary steam reforming of methane and secondary reformation in presence of added air, after undergoing a conventional shift reaction is comprised predominantly of hydrogen with lesser amounts of CO.sub.2 and nitrogen constituting &gt;98% of the total gas mixture. A typical composition, after the high temperature shift reaction may comprise (dry basis) for example:
______________________________________ Mol % ______________________________________ Hydrogen 61.0 Carbon dioxide 18.0 Nitrogen 20.0 Carbon monoxide 0.5 Methane 0.3 Argon 0.2 ______________________________________